Closed breather disconnection detection method

ABSTRACT

Disclosed is a method for detecting disconnection of a closed breather  18  separating and recovering oil mist from blow-by gas  17  extracted from an engine  1  to return the blow-by gas through a gas return pipe  19  to an intake pipe  5 . Under condition of no exhaust gas  9  recirculation being conducted, a mass flow rate of in-cylinder working gas is calculated based on a boost pressure, an intake temperature of an intake manifold  7  and a rotational frequency of the engine, and whether the calculated mass flow rate is divergent from a value detected by an air flow sensor  24  is determined. When determined to be divergent, the divergence in a last determination is compared with a current divergence; if difference between the divergences is beyond a predetermined range, the gas return pipe  19  is determined to be in disconnection from the intake pipe  5.

TECHNICAL FIELD

The present invention relates to a method for detecting disconnection ofa closed breather.

BACKGROUND ART

Blow-by gas leaking through a gap of a piston ring into a crankcaseduring engine compression and explosion strokes, which fills thecrankcase and a cylinder head cover in communication therewith, isrequired to be released outside. However, the crankcase and the cylinderhead cover are full of oil mist since the crankcase has therein acrankshaft, a connecting rod and the like which move fast, and thecylinder head cover in communication with the crankcase has therein arocker arm, a valve and the like which are in work.

Thus, release of the blow-by gas as it is to the atmosphere maydisadvantageously cause the oil mist mixed in the blow-by gas to be alsodischarged outside, so that provided is a closed breather (CCV: ClosedCrankcase Ventilator) having therein a filtering net or a labyrinthstructure for separation and recovery of the oil mist in the blow-bygas. The blow-by gas extracted from the cylinder head cover of theengine is passed to the closed breather for separation and recovery ofthe oil mist and is returned through a gas return pipe to an intake pipewhile the oil recovered by the closed breather is returned through anoil recovery pipe to an oil pan (not shown).

In such closed breather system, when the cylinder head cover isdismounted, for example, for maintenance of the engine, the gas returnpipe is also removed from the intake pipe. After completion of themaintenance or the like, there is a fear that only the cylinder headcover is remounted while re-connection of the gas return pipe to theintake pipe may be forgotten. Thus, obligated in U.S.A. from 2010 onwardis to detect disconnection (forgotten connection) of the gas return pipein a larger vehicle.

In case of the forgotten connection of the return pipe, air may be takenin also through a portion of the intake pipe to which the gas returnpipe is to be connected; then, even if a flow rate of air fed into theengine remained unchanged, an air flow rate (mass flow rate of air)detected by an air flow sensor would be reduced. Thus, for example,Patent Literature 1 mentioned hereinafter has suggested a technique thata normal air flow rate stored, for example, at a start of the engine ina last drive cycle (from key-on to key-off) is compared with a valuedetected by the air flow sensor in a current drive cycle underoperational conditions unchanged to determine disconnection of the gasreturn pipe when the detected value is lower than the normal air flowrate by a predetermined threshold value or more.

CITATION LIST Patent Literature

[Patent Literature 1] JP 2010-261325A

SUMMARY OF INVENTION Technical Problems

However, a value detected by the air flow sensor may vary even under theunchanged operational conditions if there is great environmental changein the current drive cycle in comparison with the last drive cycle suchas substantial change in ambient temperature due to large shift inweather or/and substantial change in atmospheric pressure due to changein altitude by move of the vehicle; thus, there is a fear that the gasreturn pipe may be erroneously determined to be in disconnection despiteof being connected or that the gas return pipe may be erroneouslydetermined to be normal despite of being in disconnection.

The invention was made in view of the above and has its object to makeit possible to accurately determine disconnection of a gas return pipein a closed breather.

Solution to Problems

The invention is directed to a method for detecting disconnection of aclosed breather which separates and recovers oil mist from blow-by gasextracted from an engine to return the blow-by gas through a gas returnpipe to an intake pipe, characterized by calculating a mass flow rate ofin-cylinder working gas on the basis of a boost pressure, an intaketemperature in an intake manifold and a rotational frequency of theengine under a condition of no exhaust gas recirculation beingconducted, determining whether the calculated mass flow rate isdivergent from a value detected by an air flow sensor or not, comparinga divergence in a last determination and a current divergence whendetermined to be divergent, and determining disconnection of the gasreturn pipe from the intake pipe when a difference between both thedivergences is beyond a predetermined range.

Under the condition of no exhaust gas recirculation being conducted, thecalculated mass flow rate of the in-cylinder working gas is to be thesame as the value detected by the air flow sensor. If there is adivergence therebetween, a cause thereof conceivable is characteristicabnormality of the air flow sensor itself due to deterioration thereofor disconnection of the gas return pipe. The disconnection of the gasreturn pipe abruptly occurs by forgotten re-connection of the gas returnpipe after completion of maintenance work or the like or disconnectionfor some reason or other of the return pipe having been incompletelyconnected to the intake pipe. Thus, if a difference between thedivergence in the last determination and the current divergence is greatbeyond the predetermined range, the gas return pipe can be determined tobe in disconnection from the intake pipe.

By contrast, despite of the calculated mass flow rate of the in-cylinderworking gas being divergent from the value detected by the air flowsensor, if the difference between the divergence in the lastdetermination and the current divergence is small within thepredetermined range, it can be determined that the divergence betweenthe calculated mass flow rate of the in-cylinder working gas and thevalue detected by the air flow sensor is caused by characteristicabnormality of the air flow sensor itself due to deterioration thereof.

Because, existing air flow sensors are of hot-wire type where electriccurrent is applied to a wire exposed to an intake air flow so as to keepa temperature of the wire constant, an air flow rate being detected byreading a value of electric current (the more the air flow rate is, themore the wire is cooled and the more the electric current is required soas to keep the temperature of the wire constant). In such air flowsensor, deterioration thereof slowly progresses with dusts graduallyaccumulated on the wire (sensing portion), which is deemed to causenot-so-great difference between the divergence in the last determinationand the current divergence.

In the invention, it is preferable that whether values detected by theboost pressure sensor and by the atmospheric pressure sensor aredivergent or not in the key-off is determined and stored and that a factof the non-divergence determination being stored is used as aprecondition.

More specifically, in the key-off, the values detected by the boostpressure sensor and by the atmospheric pressure sensor are to be thesame. If the fact of the non-divergence determination being stored isused as the precondition, it can be preliminarily confirmed that theboost pressure sensor and the atmospheric pressure sensor are normal.

In this connection, if determined and stored is a fact that valuesdetected by the boost pressure sensor and by the atmospheric pressuresensor are divergent in the key-off, then it is deemed that either ofthe boost pressure sensor or the atmospheric pressure sensor hascharacteristic abnormality.

More specifically, if the calculated mass flow rate of the in-cylinderworking gas is determined to be divergent from the value detected by theair flow sensor, the boost pressure used in calculation of the mass flowrate of the in-cylinder working gas is deemed to be erroneous and thus,it can be determined that the boost pressure sensor for detecting theboost pressure has characteristic abnormality.

By contrast, if the calculated mass flow rate of the in-cylinder workinggas is determined not to be divergent from the value detected by the airflow sensor, then the boost pressure used in calculation of the massflow rate of the in-cylinder working gas is deemed to be accuratelydetected; thus, normality of the boost pressure sensor is settled sothat it can be determined that the remainder, i.e., the atmosphericpressure sensor has characteristic abnormality.

Advantageous Effects of Invention

According to a method for detecting disconnection of a closed breatherof the invention as mentioned in the above, various excellent effectsand advantages can be obtained as follows.

(I) By comparing a calculated mass flow rate of in-cylinder working gaswith a value detected by an air flow sensor obtained under environmentalconditions unchanged, and further by comparing the divergence in thelast determination and the current divergence for exclusion of anypossible characteristic abnormality due to deterioration of the air flowsensor itself, disconnection of the gas return pipe of the closedbreather can be accurately determined with no affection by environmentalchange for example in ambient temperature and/or in atmosphericpressure.

(II) A determination way may be employed that when the calculated massflow rate of the in-cylinder working gas is determined to be divergentfrom the value detected by the air flow sensor, comparison is madebetween the divergence in the last determination and the currentdivergence; difference in both the divergences is within a predeterminedrange, the air flow sensor is determined to be deteriorated itself andhave characteristic abnormality. Then, in addition to the accuratedetermination on disconnection of the gas return pipe of the closedbreather, characteristic abnormality of the air flow sensor itself dueto deterioration thereof may be determined.

(III) A determination way may be employed that whether values detectedby a boost pressure sensor and by an atmospheric pressure sensor aredivergent or not in a key-off is determined and stored and that a factof the non-divergence determination being stored is used as aprecondition; then, normality of the boost pressure sensor and of theatmospheric pressure sensor can be preliminarily confirmed, which canfurther enhance the reliability in the determination on disconnection ofthe gas return pipe of the closed breather.

(IV) A determination way may be employed that when a determination thatthe values detected by the boost pressure sensor and by the atmosphericpressure sensor are divergent in the key-off is stored and if thecalculated value of the mass flow rate of the in-cylinder working gas isdetermined to be divergent from the value detected by the air flowsensor, the boost pressure sensor for detecting the boost pressure isdetermined to have characteristic abnormality. Then, in addition to theaccurate determination on disconnection of the gas return pipe in theclosed breather, the boost pressure sensor for detecting the boostpressure may be determined to have characteristic abnormality.

(V) A determination way may be employed that when a determination thatthe values detected by the boost pressure sensor and by the atmosphericpressure sensor are divergent in the key-off is stored and if thecalculated value of the mass flow rate of the in-cylinder working gas isdetermined not to be divergent from the value detected by the air flowsensor, then the atmospheric pressure sensor for detecting atmosphericpressure is determined to have characteristic abnormality. Then, inaddition to the accurate determination on disconnection of the gasreturn pipe in the closed breather, the atmospheric pressure sensor fordetecting atmospheric pressure may be determined to have characteristicabnormality.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic view showing an embodiment of the invention.

DESCRIPTION OF EMBODIMENT

Next, an embodiment of the invention will be described in conjunctionwith the drawing.

FIG. 1 shows the embodiment of the invention in which reference numeral1 denotes an engine with a turbocharger 2. The turbocharger 2 has acompressor 2 a to which intake air 4 guided from an air cleaner 3 ispassed through an intake pipe 5. The intake air 4 pressurized by thecompressor 2 a is passed to an intercooler 6 for cooling and then isguided to an intake manifold 7 where the air is distributed torespective cylinders 8 of the engine 1.

The exhaust gas 9 discharged through the cylinders 8 of the engine 1 isfed through an exhaust manifold 10 to a turbine 2 b of the turbocharger2. The exhaust gas 9 having driven the turbine 2 b is discharged throughan exhaust pipe 11 to outside of the vehicle.

An end of the exhaust manifold 10 in a direction of arrangement of thecylinders 8 is connected to a portion of the intake pipe 5 downstream ofthe intercooler 6 through an EGR pipe 12 so as to extract part of theexhaust gas 9 from the exhaust manifold 10 and guide the same as EGR gas9′ into the intake pipe 5.

The EGR pipe 12 is provided with an EGR valve 13 for selectiveopening/closing of the EGR pipe 12 and with an EGR cooler 14 for coolingof the EGR gas 9′ flowing through the EGR pipe 12. The EGR cooler 14 isadapted to lower the EGR gas 9′ in temperature through heat exchangewith cooling water guided from the engine 1.

Extracted from the cylinder head cover 15 of the engine 1 through a gasvent pipe 16 is blow-by gas 17 which is passed through a closed breather18 for separation and recovery of oil mist and is returned through a gasreturn pipe 19 to the intake pipe 5 while the oil recovered in theclosed breather 18 is returned through an oil recovery pipe 20 to an oilpan (not shown).

Further, a temperature of the intake air 4 passed through theintercooler 6 is detected by an intercooler outlet temperature sensor21. A temperature of the intake air 4 mixed with the EGR gas 9′ and tobe guided to the intake manifold 7 is detected by an intake manifoldtemperature sensor 22. A temperature and a mass flow rate of the intakeair 4 (fresh air) guided from the air cleaner 3 are detected by anintake temperature sensor 23 and an air flow sensor 24, respectively.

Here, for convenience of explanation, the intake temperature sensor 23and the air flow sensor 24 are separately disclosed. However, actuallythe temperature of the intake air 4 (fresh air) is required forobtaining the mass flow rate of the intake air 4, so that the intaketemperature sensor 23 is usually built in the air flow sensor 24.

A boost pressure of the intake air 4 to be guided to the intake manifold7 is detected by a boost pressure sensor 25 arranged in the intake pipe5 at a position adjacent to the intake manifold 7, and an atmosphericpressure is detected by an atmospheric pressure sensor 27 in acontroller 26 constituting an engine controlling computer (ECU:Electronic Control Unit).

Values detected by the intercooler outlet temperature sensor 21, theintake manifold temperature sensor 22, the intake temperature sensor 23,the air flow sensor 24, the boost pressure sensor 25 and the atmosphericpressure sensor 27 are inputted to the controller 26. Also inputted tothe controller 26 are values detected by an accelerator sensor (notshown) for detection of an accelerator opening degree as load of theengine 1 and by an rotation sensor 28 for detection of a rotationalfrequency of the engine 1.

In the controller 26, under a condition of no exhaust gas 9recirculation being conducted, a mass flow rate of in-cylinder workinggas is calculated on the basis of the boost pressure, the intaketemperature of intake manifold 7 and the rotational frequency of theengine, and whether the calculated mass flow rate is divergent from thevalue detected by the air flow sensor 24 or not is determined. If thevalues are divergent, “abnormality” determination is made; if not,“normality” determination is made.

A volumetric summation of the cylinders 8 of the engine 1 is fixed and avolumetric flow rate of the working gas taken in by the engine 1 perrotation thereof is already ascertained so that, if the boost pressure,the intake temperature of the intake manifold 7 and the rotationalfrequency of the engine are revealed, then the mass flow rate of theworking gas per unit time (consistent with unit time in detection by theair flow sensor 24) can be calculated, using gas state equation.

In comparison of such calculated mass flow rate of the in-cylinderworking gas with the value detected by the air flow sensor 24, apreposition is that no exhaust gas 9 recirculation is conducted. Thus,in the embodiment, the above-mentioned determination is made at coldstartup of the engine 1 which has been stopped for more than apredetermined time and is stone-cold. Specifically, at the cold startup,a warm-up mode is applied; i.e., warm-up of the engine 1 is usuallyprioritized with the EGR valve 13 being closed, and the exhaust gas 9 isnot recirculated. Thus, it is reasonable to conduct the above-mentioneddetermination at such cold startup.

Whenever the calculated mass flow rate of the in-cylinder working gas isdetermined to be divergent from the value detected by the air flowsensor 24, a divergence between the values is calculated and stored; andat the same time, a divergence in a last determination is compared withthe current divergence. Then, if a difference between both thedivergences is great beyond a predetermined range, “abnormality”determination is made; if difference between both the divergences issmall within the predetermined range, “normality” determination is made.

Further, in the controller 26, whether the values detected by the boostpressure sensor 25 and by the atmospheric pressure sensor 27 aredivergent or not in the key-off is determined and stored. If the valuesare divergent, “abnormality” determination is made; if not, “normality”determination is made.

Then, in the controller 26, on the basis of the“normality”/“abnormality” determinations on the above-mentioned threeitems, the “normality”/“abnormality” determinations on the three itemsare compared in light of an abnormality establishment table as shown inTable 1 to thereby determine whether there is disconnection of the gasreturn pipe 19 or characteristic abnormality of the air flow sensor 24,the boost pressure sensor 25 or the atmospheric pressure sensor 27 inthe closed breather system.

TABLE 1 EVENT COMPARISON OF CALCULATED COMPARISON OF COMPARISON BETWEENMASS FLOW RATE OF IN- DIVERGENCE IN VALUES DETECTED BY CYLINDER WORKINGGAS LAST DETERMINATION BOOST PRESSURE SENSOR WITH VALUE DETECTED WITHCURRENT AND BY ATMOSPHERIC FAILURE BY AIRFLOW SENSOR DIVERGENCE PRESSURESENSOR DISCONNECTION OF X X ◯ CLOSED BREATHER CHARACTERISTIC X ◯ ◯ABNORMALITY (DETERIORATION) OF AIR FLOW SENSOR CHARACTERISTIC X — XABNORMALITY OF BOOST PRESSURE SENSOR CHARACTERISTIC ◯ — X ABNORMALITY OFATMOSPHERIC PRESSURE SENSOR ◯: NORMALITY X: ABNORMALITY —: NO RELATION

Specifically, under condition of cold startup or other where no exhaustgas 9 recirculation is conducted, the calculated mass flow rate of thein-cylinder working gas is to be the same as the value detected by theair flow sensor 24. If there is a divergence therebetween, a causethereof conceivable is characteristic abnormality of the air flow sensor24 itself due to deterioration thereof or disconnection of the gasreturn pipe 19. The disconnection of the gas return pipe 19 abruptlyoccurs by forgotten re-connection of the gas return pipe 19 aftercompletion of maintenance work or the like or disconnection for somereason or other of the gas return pipe having been incompletelyconnection to the intake pipe 5. Thus, if a difference between thedivergence in the last determination and the current divergence is greatbeyond the predetermined range, the gas return pipe 19 can be determinedto be in disconnection from the intake pipe 5.

By contrast, despite of the calculated mass flow rate of the in-cylinderworking gas being divergent from the value detected by the air flowsensor 24, if the difference between the divergence in the lastdetermination and the current divergence is small within thepredetermined range, it can be determined that the divergence betweenthe calculated mass flow rate of the in-cylinder working gas and thevalue detected by the air flow sensor 24 is caused by the characteristicabnormality of the air flow sensor 24 itself due to deteriorationthereof.

Because, existing air flow sensors 24 are of hot-wire type whereelectric current is applied to a wire exposed to an intake air 4 flow soas to keep a temperature of the wire constant, an air flow rate beingdetected by reading a value of electric current (the more the air flowrate is, the more the wire is cooled and the more the electric currentis required so as to keep the temperature of the wire constant). In suchair flow sensor, deterioration thereof slowly progresses with dustsgradually accumulated on the wire (sensing portion), which is deemed tocause not-so-great difference between the divergence in the lastdetermination and the current divergence.

In this regard, in order to conduct such determination, it is necessaryto confirm reliability of the boost pressure sensor 25 for detection ofthe boost pressure required for calculation of the mass flow rate of thein-cylinder working gas. Thus, whether values detected by the boostpressure sensor 25 and by the atmospheric pressure sensor 27 aredivergent or not in a key-off is determined and stored, and a fact ofthe non-divergent determination being stored is used as a precondition.

Specifically, the value detected by the boost pressure sensor 25 is tobe the same as the value detected by the atmospheric pressure sensor 27in the key-off. If the non-divergent determination being stored is usedas a preposition, the normality of the boost pressure sensor 25 can bepreliminarily confirmed. In this case, the normality of the atmosphericpressure sensor 27 can be also confirmed.

In this connection, if determined and stored is a fact that the valueddetected by the boost pressure sensor 25 and by the atmospheric pressuresensor 27 are divergent in the key-off, then it is deemed that either ofthe boost pressure sensor 25 or the atmospheric pressure sensor 27 hascharacteristic abnormality.

Specifically, if the calculated mass flow rate of the in-cylinderworking gas is determined to be divergent from the value detected by theair flow sensor 24, the boost pressure used in calculation of the massflow rate of the in-cylinder working gas is deemed to be erroneous andthus, it can be determined that the boost pressure sensor 25 fordetecting the boost pressure has characteristic abnormality.

By contrast, if the calculated mass flow rate of the in-cylinder workinggas is determined not to be divergent from the value detected by the airflow sensor 24, then the boost pressure used in calculation of the massflow rate of the in-cylinder working gas is deemed to be accuratelydetected; thus, normality of the boost pressure sensor 25 is settled sothat it can be determined that the reminder, i.e., the atmosphericpressure sensor 27 has characteristic abnormality.

More strictly speaking, also required is confirmation on reliability ofthe intake manifold temperature sensor 22 for detecting the intaketemperature of the intake manifold 7 required for calculation of themass flow rate of the in-cylinder working gas. However, intake-airtemperature sensors, which have not-so-severe temperature rangerequirements, generally have higher reliability, so that the intake-airtemperature sensors are not added in the abnormality establishment tablefor the controller 26 (see Table 1).

However, reliability of the intake manifold temperature sensor 22 may beconfirmed as needs demand. For example, at the cold startup where themass flow rate of the in-cylinder working gas is calculated, normaldetection may be settled by confirming that the value detected by theintake temperature sensor 23 is the same as those of the intercooleroutlet temperature sensor 21 and the intake manifold temperature sensor22.

Specifically, at the cold startup of the engine 1 where the engine 1 isstone-cold and warm-up control is prioritized with the EGR valve 13being closed, each of the temperatures detected by the intercooleroutlet temperature sensor 21 and by the intake manifold temperaturesensor 22 is to be the same as ambient air temperature, and the intaketemperature sensor 23 for detecting the ambient air temperature is tohave the same detected value.

In addition, the rotational frequency of the engine used for calculationof the mass flow rate of the in-cylinder working gas is detected by therotation sensor 28; and generally the engine rotational frequency isalso detected by a crank angle sensor (not shown) in addition to theillustrated rotation sensor 28 (main rotation sensor). These sensors arenot added in the abnormality establishment table (see Table 1) for thecontroller 26 since highly reliable rotational frequency is easily usedfor the calculation. However, of course, agreement of the rotationalfrequency detected by the crank angle sensor with the value detected bythe rotation sensor 28 may be confirmed to settle the normal detectionof the engine rotational frequency.

As is clear from the foregoing, according to the above embodiment,comparison is made between the calculated mass flow rate of thein-cylinder working gas and the value detected by the air flow sensor 24which are obtained in the environmental condition unchanged, and furthercomparison is made between the deviation in the last determination andthe current deviation for exclusion of any possible characteristicabnormality due to deterioration of the air flow sensor 24 itself, sothat disconnection of the gas return pipe 19 in the closed breathersystem can be accurately determined with no affection by environmentalchange for example in ambient temperature and/or in atmosphericpressure. Moreover, in addition to the accurate determination on thedisconnection of the gas return pipe 19, characteristic abnormality dueto deterioration of the air flow sensor 24 itself may be determined.Further, for conducting such determination, the normality of the boostpressure sensor 25 and of the atmospheric pressure sensor 27 may bepreliminarily confirmed to further enhance the reliability indetermination of the disconnection of the gas return pipe 19 in theclosed breather system.

Further, especially in the embodiment, in addition to the accuratedetermination on disconnection of the gas return pipe 19 in the closedbreather system, characteristic abnormality of the boost pressure sensor25 for detecting the boost pressure or of the atmospheric pressuresensor 27 for detecting the atmospheric pressure may be determined.

It is to be understood that a method for detecting disconnection of aclosed breather according to the invention is not limited to the aboveembodiment and that various changes and modifications may be madewithout departing from the scope of the invention.

REFERENCE SIGNS LIST

-   -   1 engine    -   4 intake air    -   5 intake pipe    -   7 intake manifold    -   8 cylinder    -   17 blow-by gas    -   18 closed breather    -   19 gas return pipe    -   23 intake temperature sensor    -   24 air flow sensor    -   25 boost pressure sensor    -   27 atmospheric pressure sensor

The invention claimed is:
 1. A method for detecting disconnection of aclosed breather which separates and recovers oil mist from blow-by gasextracted from an engine to return the blow-by gas through a gas returnpipe to an intake pipe, comprising: calculating a mass flow rate ofin-cylinder working gas on the basis of a boost pressure, an intaketemperature in an intake manifold, and a rotational frequency of theengine under a condition of no exhaust gas recirculation beingconducted, determining whether the calculated mass flow rate isdivergent from a value detected by an air flow sensor or not, comparinga divergence in a last determination and a current divergence whendetermined to be divergent, and determining disconnection of the gasreturn pipe from the intake pipe when a difference between both thedivergences is beyond a predetermined range, wherein whether valuesdetected by a boost pressure sensor and by an atmospheric pressuresensor are divergent or not in a key-off state is determined and storedand a fact of non-divergence determination being stored is used as aprecondition.
 2. The method for detecting disconnection of the closedbreather as claimed in claim 1, wherein if the calculated mass flow rateof the in-cylinder working gas is determined to be divergent from thevalue detected by the air flow sensor, the divergence in the lastdetermination is compared with the current divergence; and if thedifference between both the divergences is within the predeterminedrange, the air flow sensor is determined to have characteristicabnormality due to deterioration of the air flow sensor itself.
 3. Themethod for detecting disconnection of the closed breather as claimed inclaim 2, wherein when the values detected by the boost pressure sensorand by the atmospheric pressure sensor in the key-off state isdetermined and stored to be divergent and if the calculated mass flowrate of the in-cylinder working gas is determined to be divergent fromthe value detected by the air flow sensor, the boost pressure sensor fordetecting the boost pressure is determined to have characteristicabnormality.
 4. The method for detecting disconnection of the closedbreather as claimed in claim 2, wherein when the values detected by theboost pressure sensor and by the atmospheric pressure sensor in thekey-off state is determined and stored to be divergent and if thecalculated mass flow rate of the in-cylinder working gas is determinednot to be divergent from the value detected by the air flow sensor, theatmospheric pressure sensor for detecting the atmospheric pressure isdetermined to have characteristic abnormality.
 5. The method fordetecting disconnection of the closed breather as claimed in claim 1,wherein when the values detected by the boost pressure sensor and by theatmospheric pressure sensor in the key-off state is determined andstored to be divergent and if the calculated mass flow rate of thein-cylinder working gas is determined to be divergent from the valuedetected by the air flow sensor, the boost pressure sensor for detectingthe boost pressure is determined to have characteristic abnormality. 6.The method for detecting disconnection of the closed breather as claimedin claim 1, wherein when the values detected by the boost pressuresensor and by the atmospheric pressure sensor in the key-off state isdetermined and stored to be divergent and if the calculated mass flowrate of the in-cylinder working gas is determined not to be divergentfrom the value detected by the air flow sensor, the atmospheric pressuresensor for detecting the atmospheric pressure is determined to havecharacteristic abnormality.